The invention relates to a method for operating a longitudinal driver assistance system of a motor vehicle, which controls the speed of the motor vehicle in a pure follow mode so that a distance to a preceding vehicle as the control object remains constant, and which controls in a free-driving mode to a desired speed, and a motor vehicle with such a driver assistance system.
Such longitudinal driver assistance systems are already known in the art. They control the operation of the motor vehicle at least partly automatically through automatic braking or acceleration interventions. A simple longitudinal driver assistance system is the so-called cruise control system (CCS), also referred to as Tempomat. In this case, the driver can set a desired speed, which is then automatically maintained as closely as possible by the driver assistance system.
As an extension of this basic idea, longitudinal driver assistance systems with a so-called follow controller, frequently referred to as ACC systems (Adaptive Cruise Control), have been proposed. The driver's desired speed is here not only regulated in a free-driving mode, but it is also monitored whether a preceding vehicle is present. When this is the case, in particular within a control distance, frequently a time gap, that can optionally also be adjusted by the driver, a change is made to a pure follow mode, wherein the speed of the vehicle is adjusted so that the distance to the preceding vehicle corresponds to the control distance. The preceding vehicle, in relation to which the speed is controlled, is also referred to as control object.
The control object may be determined, for example, with a so-called plausibility method. Objects driving in front of the motor vehicle are hereby determined and evaluated based on criteria. For example, it is known to calculate a vehicle path that corresponds to a most likely future trajectory of the motor vehicle, and to exclude from consideration all objects located outside the vehicle path, wherein the closest object in the direction of travel of the vehicle along the vehicle path is then frequently selected as a the control object. The reverse process, i.e. the loss of the control object and thus the termination of the follow mode, is often referred to as deplausibilization of the control object since its plausibility as a control object drops, for example, below a threshold value, or the object is no longer detected, for example due to a lane change or a measurement error.
To deplausibilize a preceding vehicle pulling out of the travel lane of the host vehicle, the control object is lost when the preceding vehicle leaves the vehicle path. Only at this point is the vehicle accelerated again to the desired speed as comfortable as possible, after transitioning into the free-driving mode. However, this causes the following problem. When driving behind another vehicle that passes a preceding vehicle as the control object and when approaching a gap in which the passing preceding vehicle provides the own vehicle with an opportunity to pass, the own acceleration is usually so late and so sluggish that the gap of the other vehicle, the former control object, has again become too small, so that the driver of this motor vehicle changes lanes again and the own vehicle, because the distance has become smaller, clearly keeps away by braking in order to create the control distance and thus requires more time to transition to acceleration after the next passing maneuver at the next gap. This effect can repeat until there is a long gap, which is sufficient for passing the former control object even with the delayed acceleration. This does not correspond in any way to the driver's expectation because it is easier from his experience to recognize a vehicle as pulling out, whereupon he can accelerate significantly earlier, even when the preceding vehicle is still in the driver's lane.